The solid oxide fuel cell (SOFC) promises a high conversion efficiency (40 to 60%) of chemical energy to electric power with negligible pollution and is attractive for use in the co-generation of electric power. The prototype SOFCs now being marketed use yttria-stabilized zirconia (YSZ) as the oxide-ion electrolyte. This requires an operating temperature T.sub.op .apprxeq.1000.degree. C. if conventional ceramic membranes are used. The interconnect between individual cells must be stable in both the oxidizing atmosphere at the cathode and the reducing atmosphere at the anode, and at T.sub.op .apprxeq.1000.degree. C. it is necessary to use a conducting ceramic for the interconnects. However, even the ceramic of choice, Ca-doped LaCrO.sub.3, loses oxygen from the side exposed to the anodic atmosphere and gains oxygen on the side exposed to the cathodic atmosphere. This causes the interconnect membranes to warp. An operating temperature in the range 600.degree. C.&lt;T&lt;800.degree. C. could allow the use of an oxidation-resistant stainless steel or another alloy as the interconnect material. The lower operating temperature would also reduce operating costs, increase durability, extend service life, and permit more frequent cycling.
Two approaches to a T.sub.op &lt;800.degree. C. are under active consideration: (1) reduction of the thickness of the YSZ electrolyte membrane to l.ltoreq.10 .mu.m and (2) use of a solid electrolyte having an oxide-ion conductivity at or below 800.degree. C. that is comparable to that of YSZ at 1000.degree. C. The most promising traditional material for the second approach is CeO.sub.2 doped with an alkaline-earth oxide, AO, or a rare earth oxide, Ln.sub.2 O.sub.3, but reduction of Ce.sup.4+ to Ce.sup.3+ in the anodic gas introduces into the electrolyte an unwanted polaronic conduction.
There is a need for a solid electrolyte having a high oxide-ion conductivity at a reduced operating temperature, negligible electronic conductivity over a wide range of oxygen partial pressure, viz 10.sup.-22 &lt;Po.sub.2 &lt;1 atm, and stable performance over extended periods of time.
The pseudo-cubic perovskite system La.sub.1-x Sr.sub.x Ga.sub.1-y Mg.sub.y O.sub.3-0.5(x+y) is attracting increasing attention as an oxide-ion solid electrolyte competitive with yttria-stabilized zirconia; it demonstrates an oxide-ion conductivity .sigma..sub.o .gtoreq.0.10 S/cm at 800.degree. C., a negligible electronic conduction at temperatures T&lt;1000.degree. C. over a broad range of oxygen partial pressure from pure oxygen Po.sub.2 =1 atm) to moistened hydrogen (Po.sub.2.sup..about. 10.sup.-22 atm), and a stable performance over long operating periods. These superior electrical and chemical properties make it a strong candidate for use as the solid electrolyte in reduced-temperature solid oxide fuel cells (RTSOFCs) operating at or below 800.degree. C.
A typical prior art doped gallate, La.sub.0.9 Sr.sub.0.1 Ga.sub.0.8 Mg.sub.0.2 O.sub.2.85, has an ionic conductivity of 0.07-0.1 S/cm.sup.2 at 800.degree. C. However, at room temperature it always contains an undesirable nonconducting second phase LaSrGaO.sub.4. The presence of this phase, which may gradually disappear at higher temperatures, may explain the unusually rapid decrease of ionic conductivity with decreased temperature. For instance, the conductivity, measured by a 4 probe DC measurement technique, was 0.075 S/cm at 800.degree. C., but only 0.028 S/cm at 700.degree. C.